How can I get the compiler to insert the correct type in this example?

问题

this is somehow follow-up on this question How can I get rid of this "Can not be generalised" error?

given the following types

type IModel<'value, 'target when 'target :> IModel<'value, 'target>> =
    abstract value: 'value with get

type IModelSimple<'value, 'target> =
    inherit IModel<'value, IModelSimple<'value, 'target>>
    abstract ReInitWith:  #IModel<_ , _ > -> 'target

and this function returning an object expression

let rec mkModelSimple<'value, 'target> vctor value =
    {
        new IModelSimple<'value, 'target> with
            member this.value = value
            member this.ReInitWith m = mkModelSimple vctor this.value
    }

I am getting this error

Type mismatch. Expecting a
    ''target'
but given a
    'IModelSimple<'value,'target>'

in the implementation of ReInitWith.

回答1:

As the compiler reports, you are defining a function where the inferred type contains itself and would be infinite. The way to fix this is to introduce some named type that breaks the recursion. One option is to define a new record with recursive type:

type IModel<'value, 'target when 'target :> IModel<'value, 'target>> =
    abstract value: 'value with get

type IModelSimple<'value, 'target> =
    inherit IModel<'value, IModelSimple<'value, 'target>>
    abstract ReInitWith :  #IModel<_, _> -> 'target

type Model<'value, 'target> =
  { Model : IModelSimple<'value, Model<'value, 'target>> }

let rec mkModelSimple<'value, 'target> vctor value =
    { Model = 
        { new IModelSimple<'value, Model<'value, 'target>> with
              member this.value = value
              member this.ReInitWith m = 
                mkModelSimple<_, _> vctor this.value } }

Just for the record, I think this code is pretty incomprehensible. It is definitely stretching what generics can sensibly do and I'd recommend avoiding doing things like this in production.

来源：https://stackoverflow.com/questions/48813678/how-can-i-get-the-compiler-to-insert-the-correct-type-in-this-example

标签